1. Field of the Invention
This invention relates to an image reading device, and more particularly to an image reading device for optically reading the image of an original placed on a original table.
2. Description of the Related Art
In this type of image reading device, an original is put on a transparent original table and positioned in the reading area or reading section of the original table. The image of the original is illuminated by means of a light source provided below the original table and is read according to the amount of light reflected on the surface of the original. In a case where a line type image sensor having a plurality of light receiving elements arranged in a row is used to read the image, the image sensor is mounted on a carrier which is movable in the lengthwise direction (or subscanning direction) of the reading section with the line of light receiving elements being placed in parallel with a line extending in the width direction (or main scanning direction) of the reading section. In the image reading mode, the carrier is first set near one end of the original table, and is then driven to cross the reading section of the original table. The image sensor causes the plurality of light receiving elements to effect the sensing operation each time the carrier moves a preset distance, picture element data of one line are serially output in the order from one end of the line of light receiving elements to the other end thereof. All the image of the original is converted into picture element data while the image sensor is moving across the reading section, and is then stored into a memory of the image reading device.
In the prior art image reading device, limit switch 102 as is shown in FIG. 1 is used, for example, to set the position at which the operation of reading the image of the original is started. When the carrier is moved back as indicated by arrow A to come into contact with limit switch 102 mounted near one end of the original table, a detection signal is generated from limit switch 102. The image reading device sets the reading starting position in response to the detection signal with the present position of the carrier set as a reference position. That is, the reading starting position is set at a position of the image sensor which is set when the carrier is moved forward by distance L as shown by arrow B from the present position. Picture element data output from the image sensor is made effective as the picture element data of the original image after the image sensor has reached the reading starting position. Distance L is previously determined to be equal to the interval between the reading section and the image sensor with the carrier set in contact with limit switch 102. Therefore, if the present position of the carrier can be precisely detected by means of limit switch 102, the front end position of the reading section coincides with the reading starting position.
However, limit switch 102 has the following defects. That is, the mounting position of limit switch 102 may vary in the manufacturing process of the image reading device, and the mechanical operation of limit switch 102 tends to be delayed. If the present position of carrier 104 cannot be precisely detected, the reading starting position in the sub-scanning direction does not coincide with the front end position of reading section 101. In this case, image other than that of the original may be read.
In another type of image reading device, narrow rectangular mark 103 as shown in FIG. 2 may be used to set the reading starting position of the original image. Mark 103 has first and second long sides extending in the main scanning direction, a first short side set in the same position as the first long side of the reading section in the main scanning direction and extending in the sub-scanning direction, and a second short side set between the first and second long sides of the reading section in the main scanning direction and extending in the sub-scanning direction. Mark 103 is detected by means of the image sensor while the carrier is moved forwardly to the reading section as shown by narrow B after it has been moved back as indicated by arrow A. The image reading device sets the reading starting position in the sub-scanning direction by setting the present position of the carrier as a reference position when mark 103 is detected. Further, it sets the position of the first short side of mark 103 to the reading starting position in the main scanning direction. The reading starting position in the sub-scanning direction is set at the position of the image sensor which is set when the carrier is moved by preset distance L in a direction indicated by arrow B from the present position in the same manner as in the prior art case. That is, if mark 103 is precisely detected, the front end position of the reading section can be set to coincide with reading starting position in the sub-scanning direction. Further, if the first short side of mark 103 is precisely detected, the position of the first long side of the reading section can be set to coincide with the reading starting position in the main scanning direction. In this case, the image reading device derives, as effective picture element data of the original image, picture element data which is included in the picture element data output from the image sensor after the image sensor has reached the reading starting position and which is output from the light receiving elements of a preset number corresponding to the width of the reading section.
Now, the operation of the detecting mark 103 is explained with reference to FIG. 3A. Image sensor 105 reads the image of one line each time carrier 104 is moved by a preset distance. For this reason, main scanning lines P1, P2, P3 - - - are placed at a regular interval in FIG. 3A. When image sensor 105 has reached a position corresponding to main scanning lines P5, picture element data indicating the color of mark 103, for example, black is output from a preset number of or more successive light receiving elements. When mark 103 is detected by such picture element data, the reading starting position in the sub-scanning direction is set to a position of main scanning line Pi which is placed at distance L from main scanning line P5 in the forward direction. At the same time, the reading starting position in the main scanning direction is set to a position of that one of the line of successive light receiving elements which corresponds to one end position X of mark 103.
The mounting position of parts of the image reading device may vary in the manufacturing process and may be displaced due to vibration of the device occurring in operation. For example, where image sensor 105 is mounted at an angle with respect to the main scanning direction, main scanning lines P1, P2, P3 - - - may be set to extend in a right upper direction as shown in FIG. 3B or in a right lower direction as shown in FIG. 3C. In the case of FIG. 3B, the reading starting position in the main scanning direction will be erroneously set to position X'. Likewise, in the case of FIG. 3C, the reading starting position in the sub-scanning direction will be erroneously set to a position separated by distance L from main scanning line P4 in a forward direction. In particular, the reading starting position in the main scanning direction may significantly vary with the slightest inclination of image sensor 105. For this reason, it is impossible to disregard the setting error of the reading starting position in the main scanning direction.